Matt, I found that there is some sort of threshold below which tannins were extracted into my brews. I had been using 2 brix as the cutoff for about a half dozen brews and kept finding a very mild tannic astringency in those beers. Moving the cutoff gravity to 3 brix solved that problem, but my problem was that I was too focused on kettle volume and the fact that I still had a bunch of water in the HLT.
I use RO water with alkalinity of nearly zero alkalinity and it would only take a drop or two of acid to drop its pH well below 6. I don't runoff for super long periods. I'm usually done with the runoff in about 45 minutes. That is similar to the time I was taking before this issue presented itself, but I did switch from my LHBS' crappy milling to milling my own grist with a MM-2.2 that is set at 0.035" gap. It is a very fine crush, but my pre-conditioning of the malt prior to the crush is leaving a large percentage of intact husk for filtering. My RIMS flow is still pretty good.
The fact that 90% of my sugars do make it into the kettle is encouraging. It's just that I had not run into an inability to run the entire calculated sparging volume through the grist before. I had less volume of wort, but at higher gravity. Now I will conduct operations more like Black Sands and monitor the runoff gravity more closely and just end runoff as the refractometer indicates and add any remaining water in the HLT directly to the kettle to meet my pre-boil volume goal. That should avoid the potential for tannin extraction.
Thanks for the guidance on this issue. I figured that this was a solution, but I'd never heard of anyone doing it that way.